Abstract

Cluster models of condensed systems are often used to simulate the core-level spectra obtained with X-ray Photoelectron Spectroscopy, XPS, or with X-ray Absorption Spectroscopy, XAS, especially for near edge features.

The theoretical research in this project has been directed toward the interpretation of core-level spectroscopies for systems relevant to the project. For the initial efforts, the focus of our theoretical simulations has been the interpretation of laboratory and synchrotron X-Ray Photoemission Spectra, XPS. In more recent efforts, an increasing emphasis has been placed on developing transparent understandings of X-Ray Adsorption Spectra, XAS . For the XAS, the principal concern is for the near-edge features, either just below or just above, an ionization limit or edge, which are described as Near-Edge X-Ray Adsorption Fine Structure, NEXAFS. In particular, a priority hasmore » involved the analysis and interpretation of XPS and NEXAFS spectra, especially of Fe and U systems, as measured by our PNNL collaborators. The overall objective of our theoretical studies is to establish connections between features of the spectra and their origin in the electronic structure of the materials. The efforts for the analysis of XPS have been reviewed in a paper by the PI, C. J. Nelin, and E. S. Ilton from PNNL on “The interpretation of XPS spectra: Insights into materials properties”, Surf. Sci. Reports, 68, 273 (2013). Two materials properties of special interest have been the degree of ionicity and the character of the covalent bonding in a range of oxides formed with transition metal, lanthanide, and actinide cations. Since the systems treated have electrons in open shells, it has been necessary to determine the energetics and the character of the angular momentum coupling of the open shell electrons. In particular, we have established methods for the treatment of the “intermediate coupling” which arises when the system is between the limit of Russell-Saunders multiplets, and the limit of j-j coupling where the spin-orbit splittings of single electrons dominate. A recent paper by the PI, and M. J. Sassi, and K. M. Rosso, (both at PNNL) “Intermediate Coupling For Core-Level Excited States: Consequences For X-Ray Absorption Spectroscopy”, J. Elec. Spectros. and Related Phenom., 200, 174 (2015) describes our first application of these methods. As well as applications to problems and materials of direct interest for our PNNL colleagues, we have pursued applications of fundamental theoretical significance for the analysis and interpretation of XPS and XAS spectra. These studies are important for the development of the fields of core-level spectroscopies as well as to advance our capabilities for applications of interest to our PNNL colleagues. An excellent example is our study of the surface core-level shifts, SCLS, for the surface and bulk atoms of an oxide that provides a new approach to understanding how the surface electronic of oxides differs from that in the bulk of the material. This work has the potential to lead to a new key to understanding the reactivity of oxide surfaces. Our theoretical studies use cluster models with finite numbers of atoms to describe the properties of condensed phases and crystals. This approach has allowed us to focus on the local atomistic, chemical interactions. For these clusters, we obtain orbitals and spinors through the solution of the Hartree-Fock, HF, and the fully relativistic Dirac HF equations. These orbitals are used to form configuration mixing wavefunctions which treat the many-body effects responsible for the open shell angular momentum coupling and for the satellites of the core-level spectra. Our efforts have been in two complementary directions. As well as the applications described above, we have placed major emphasis on the enhancement and extension of our theoretical and computational capabilities so that we can treat complex systems with a greater range of many-body effects. Noteworthy accomplishments in terms of method development and enhancement have included: (1) An improvement in our treatment of the large matrices that must be handled when many-body effects are treated. (2) Improvements and extensions of our capabilities for the calculation of the intensities of XPS and XAS transitions. And (3) ongoing development of flexible programs for the visualization of the theoretical spectra so that they can be compared with experiment. Our efforts on applications and methodology for these and related topics will continue under a sub-contract to PNNL.« less

The metal structural environments in macroscale and nanoscale Zn xNi 1–xO solid solutions were examined using X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS). XRD demonstrates that solid solutions form for both macroscale (bulk) and nanoscale crystallites, and that the lattice parameter increases linearly as the amount of zinc increases, an indication of a homogeneous solid solution. XAS for both the bulk material and the nanoparticles reveals that the zinc atoms are incorporated into the rocksalt lattice and do not form zinc oxide clusters. The X-ray absorption near edge spectroscopy (XANES) of the Zn k-edge regionmore » in the solid solution is similar to the Ni k-edge region of NiO, and not the Zn k-edge region of ZnO. XPS confirms that solid solutions are formed; Auger parameters for zinc are consistent with a different geometry than the tetrahedral coordination of wurtzite ZnO. Nanoscaled solid solutions show evidence of a lattice contraction relative to macroscale solutions of the same concentration. While the contraction persists across the entire concentration range, the nanoparticle lattice parameter approaches the bulk Zn xNi 1–xO value as the concentration of zinc increases to predict ZnO rocksalt lattice parameters that are in agreement with observed ZnO data.« less

X-ray absorption near-edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) spectroscopies are tools in widespread use for providing detailed local atomic structure, oxidation state, and magnetic structure information for materials and organometallic complexes. The analysis of these spectra for transition-metal L-edges is routinely performed on the basis of ligand-field multiplet theory because one- and two-particle mean-field ab initio methods typically cannot describe the multiplet structure. Here we show that multireference configuration interaction (MRCI) calculations can satisfactorily reproduce measured XANES spectra for a range of complex iron oxide materials including hematite and magnetite. MRCI Fe L2,3-edge XANES and XMCD spectramore » of Fe(II)O6, Fe(III)O6, and Fe(III)O4 in magnetite are found to be in very good qualitative agreement with experiment and multiplet calculations. Point-charge embedding and small distortions of the first-shell oxygen ligands have only small effects. Oxygen K-edge XANES/XMCD spectra for magnetite investigated by a real-space Green’s function approach complete the very good qualitative agreement with experiment. Material-specific differences in local coordination and site symmetry are well reproduced, making the approach useful for assigning spectral features to specific oxidation states and coordination environments.« less

We have measured polarization-dependent x-ray-absorption spectra (XAS) at the Cu {ital K} edge in magnetically-aligned powder samples of La{sub 2}CuO{sub 4}, Nd{sub 2}CuO{sub 4}, Bi{sub 2}CuO{sub 4}, Ca{sub 0.85}Sr{sub 0.15}CuO{sub 2}, and SrCuO{sub 2}, all of which contain Cu coordinated by four coplanar O nearest neighbors. We have also measured Cu 2{ital p}{sub 3/2} x-ray photoelectron spectra (XPS) for all except the last compound. The measurements have been analyzed in terms of unrestricted Hartree-Fock calculations for CuO{sub 6}{sup 10{minus}} and CuO{sub 12}{sup 22{minus}} clusters, with energies and Coulomb parameters, including interactions with the core hole, for the Cu 3{ital d}{submore » {ital x}}{sup 2}{minus}{ital y}{sup 2}, 4{ital s}, and 4{ital p} orbitals determined in an {ital ab} {ital initio} fashion. It is well known that the intensity ratio of the two XPS peaks is sensitive to the degree of hybridization between Cu 3{ital d}{sub {ital x}}{sup 2}{minus}{ital y}{sup 2} and O 2{ital p}{sub {sigma}} orbitals in the ground state. We show both by empirical correlations and model calculations that the relative shift of the continuum absorption threshold for {cflx {epsilon}}{parallel}{bold z} (where {bold z} is perpendicular to the plane of the CuO{sub 4} units) and the shift of the main (3{ital d}{sup 10}{ital L}) XPS peak are also useful measures of the covalency. The Cu 4{ital p}{sub {ital z}}-like quasibound state observed in XAS is sensitive as well to 4{ital p}{sub {ital z}--}O 2{ital p} hybridization. In the cluster calculations, good agreement with the experimental features is obtained with use of a single adjustable parameter, the energy of the O 2{ital p}{sub {sigma}} level (measured relative to Cu 3{ital d}{sub {ital x}}{sup 2}{minus}{ital y}{sup 2}). We show that this energy varies linearly with the difference in Madelung energies for the O and Cu sites.« less

X-ray absorption spectroscopy (XAS) is a powerful tool that can provide physical insights into element-specific chemical processes and reactivities. Although relativistic time-dependent density functional theory (TDDFT) has been previously applied to model the L-edge region in XAS, there has not been a more comprehensive study of the choices of basis sets and density functional kernels available for variational relativistic excited state methods. In this work, we introduce the implementation of the generalized preconditioned locally harmonic residual algorithm to solve the complex-valued relativistic TDDFT for modeling the L-edge X-ray absorption spectra. Further, we investigate the L 2,3-edge spectra of a seriesmore » of molecular complexes using relativistic linear response TDDFT with a hybrid iterative diagonalization algorithm. A systematic error analysis was carried out with a focus on the energetics, intensities, and magnitude of L 2–L 3 splitting compared to experiments. Additionally, the results from relativistic TDDFT calculations are compared to those computed using other theoretical methods, and the multideterminantal effects on the L-edge XAS were investigated.« less